Conventional color printers typically have print resolutions of 300, 600, 1200 or higher dots per inch (DPI). These printers generally produce color output through use of separate ink pens for each color. In the specification and claims that follow, references to pens includes alternate color-delivery mechanisms such as ribbons, powder, wax, spray nozzles, and the like, in which pixels or microdots (sub-pixels) of ink ,are transferred or blended on tangible mediums such as paper. The terms microdots and droplet are used interchangeably herein, as are the terms pixel and dot. Additionally, the technology disclosed herein, as far as applicable, is intended to include output to intangible mediums such as computer displays.
Note that it is assumed that a printer is printing at 300 DPI (or 600 DPI when noted), and that values referenced herein are for exemplary purposes only. Although values may correspond to printing at 300 DPI resolution, claimed embodiments apply to other resolutions as well. This invention applies to printing techniques where output pixels are formed of more than one sub-element, such as microdots or by pixel aggregation to form lower-resolution macro-pixels.
Much research has gone into improving print quality while also keeping-low the cost of the printer. One method of reducing costs is to provide basic colors (e.g. red/green/blue or cyan/yellow/magenta), and then generate other colors by arranging printed pixels to give the appearance of color shades and continuous tones (e.g. dithering). Except for certain print processes which blend each output pixels together (e.g. dye-sublimation or thermal-wax), color printing process is based on placing discrete dots on a page so that the net effect of the colors is the desired color to be generated. The visibility of each dot depends on the image being printed, the printing substrate, and the characteristics of the pen used to deliver the ink. To minimize dot visibility, different dithering patterns have been developed to optimize dot arrangement to minimize graininess. Some methods also use differing-sized dots to improve the continuous-seeming nature of the output.
A pixel printed to a page is composed of at least one microdot (droplet) of ink, the droplet being the smallest amount of ink a pen can output. Microdots are combined to generate pixels of differing weights (saturation). The number of droplets per pixel can be represented by the variable N. For example, for a mid-weight dot size, 6 microdots of ink may be sprayed to form the printed dot.
A problem faced by prior art printing techniques is that irrespective of the pattern or dot size used to generate an output image, the output driver (embedded in hardware or software) assumes that the quality of the ink transfer mechanism is consistent across each pen. For example, the controlling software/driver in an inkjet printer assumes each pen ejects the same amount of ink when producing a microdot. There is no compensation for pen-differences due to manufacturing variations. That is, due to such variations, the same type of pen (e.g. having nominal 8 picoliter microdots) may have slightly differing ink transfer mechanisms, resulting in different droplet sizes (e.g. 7 or 9 picoliters). Thus, a pen having heavier microdots, say 9 picoliters appears to produce more saturated output than a lighter 7 picoliter pen, even though both pens are expected to be producing 8 picoliter microdots. Preferred embodiments compensate for differing pens at the microdot level.
A preferred embodiment includes a printer having one or more pens for printing some number of pixels per inch on an output medium. Each pen has an associated characteristic drop volume to indicate the actual size of droplets expelled from the pen, as opposed to the nominally expected droplet volume. Preferred printers may print a range of microdots per pixel. In the following description, it is assumed that the printer is an inkjet-type printer capable of outputting between zero and 12 microdots per pixel. The number of possible microdots per pixel corresponds to differing levels of ink saturation. This range can be grouped into three luminosity levels by use of 3 different droplet patterns (e.g. 1 droplet, 3 droplets, 12 droplets).
The number of droplets per pixel for each of three saturation levels can be represented by the variables N1, N2, and N3. The values for N1, N2, and N3 can vary from zero to 12. For example, N1 can be 1, N2 can be selected from the group consisting of 1, 2, 3, and 4, and N3 can be selected from the group consisting of 3,4,5,6,7.8,9, 10, 11, and 12. Such use of three levels (four including zero) allows saturation to be represented using just two binary digits, thus speeding print processing.
Additionally, by storing a pen's actual droplet volume in a memory associated with each pen, the printer is able to adjust each droplet pattern in accord with the pen's characteristics. Thus, a user is no longer required to attempt manual compensation via driver-based saturation controls, or by modifying the original image. If the print driver is running on a computer, preferred embodiments include a graphical user interface to make hue shift, saturation and other print-time adjustments. Such print-time adjustments are in addition to automatic compensation for pen characteristics, and may further adjust the number of microdots per pixel, or the placement of the droplets within a pixel. Note that automatic pen compensation may also include adjusting for media (paper) type.
The foregoing and other features and advantages of the preferred embodiment of the present invention is more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.